Accelerated High Temperature Performance Evaluation of Superalloys for Process Optimization and Remaining Life Assessment

Creep strength and fracture resistance are two properties that are critical in the selection and optimization of high temperature materials. These same properties may be progressively impaired by service exposure and are, therefore, key to the assessment of remaining life of high temperature components. In recent years, a methodology based on accelerated measurements of these properties has been developed. The two critical properties are decoupled in this approach which allows a clearer separation of the effects of microstructural evolution, damage development and environmental attack. The evaluation of creep strength is illustrated using extensive data on conventionally cast (IN738), directionally solidified (GTD111) and monocrystalline (CMSX-4) superalloys. It is shown that the properly defined creep strength is not sensitive to section size for any of the alloy classes. The well known effect of section size on rupture life of conventionally cast alloys is thus an effect on fracture resistance rather than creep strength. The comparisons with long term creep rupture test data show good agreement in all cases indicating that the accelerated testing is quite capable of yielding both comparative and design data. With the added advantage of a separate fracture resistance criterion the new methodology should be broadly useful.